I might add Bas, personal past somewhat short of an EA but considerable practical and educational tax background.....
Asset valuation taxable for Re characterization at the time of the conversion event.
The other comments/suggestions beat around the edge of.....
If you expect more than a 12% return on the asset you wish to use to pay your liability then your are foolish not to use another vehicle to pay the tax bill. IRS will work out to 12% (penalties and interest) up to $25K in tax liability. If you have substantial assets, they may be demand payment rather than almost automatic payment plan.
The numbers you banter make me think you can access even lower cost of money so why even consider.....other than Roth conversion which would be tremendously advantageous under your assumed future performance.